One of the primary steps in the fabrication of modem semiconductor devices is the formation of a thin film on a semiconductor substrate by chemical reaction of gases. Such a deposition process is referred to as chemical vapor deposition or CVD. Conventional thermal CVD processes supply reactive gases to the substrate surface where heat-induced chemical reactions take place to produce a desired film.
An alternative method of depositing layers over a substrate includes plasma enhanced CVD (PECVD) techniques. Plasma enhanced CVD techniques promote excitation and/or dissociation of the reactant gases by the application of radio frequency (RF) energy to a reaction zone near the substrate surface, thereby creating a plasma. The high reactivity of the species in the plasma reduces the energy required for a chemical reaction to take place, and thus lowers the temperature required for such CVD processes as compared to conventional thermal CVD processes. The relatively low temperature of some PECVD processes helps semiconductor manufacturers lower the overall thermal budget in the fabrication of some integrated circuits.
One commonly known PECVD process is referred to as a single frequency process in which a plasma is formed by applying high frequency RF energy (e.g., 13.56 MHz) to one of two electrodes positioned near the reaction zone. Another well known PECVD process is referred to as a mixed frequency processes or a dual frequency process. In a mixed frequency PECVD process, both high and low frequency RF energy (e.g., one 13.56 MHz signal and one signal less than 1 MHz) is applied to one or more electrodes positioned near the reaction zone.
One type of material that semiconductor manufacturers commonly deposit using PECVD techniques is silicon nitride. Silicon nitride films are used for a variety of different purposes in integrated circuits. Two common applications for silicon nitride films in the front end processing of integrated circuits include the formation of spacer structures around transistor gates and the formation of contact etch stop layers, such as the barrier layer between a premetal dielectric layer and the semiconductor substrate.
As semiconductor device geometries have decreased in size over the years, semiconductor manufacturers are faced with new challenges that must be overcome in order to develop robust, high yield manufacturing processes for the manufacture of integrated circuits. Because of these challenges, sometimes substrate processing techniques that were used successfully in certain previous integrated circuit manufacturing processes are not effective in newer fabrication processes. For example, single frequency, high temperature (e.g., temperatures above 500xc2x0 C.) silicon nitride films have been successfully used for barrier layers to premetal dielectric layers in a number of different integrated circuits. Such layers have been proven to have low hydrogen content, good step coverage and a relatively low thermal budget as compared to conventional low pressure thermal CVD nitride layers. At least one semiconductor manufacturer has found, however, that such layers may have limitations that make them impractical for use with certain integrated circuits that have relatively complex device fabrication requirements.
As an example, reference is made to FIG. 1, which is a simplified cross-sectional view of a partially formed integrated circuit 10. Partially formed integrated circuit 10 includes a substrate 12 having a shallow trench isolation (STI) structure 14 formed therein. A transistor gate 16 is formed over the substrate and a silicon nitride barrier layer 18 is formed over the gate prior to the formation of an overlying premetal dielectric layer 20, such as a phosphosilicate glass (PSG) layer. Also shown in FIG. 1 are contact holes 22 which are etched through PSG layer 20. While not shown in FIG. 1, contact holes 22 will be further etched through silicon nitride barrier layer 18 at a later substrate processing step.
It has been reported that seams 24, 26 and 28 may form in silicon nitride layer 18 in areas 30, 32 and 34, respectively, where layer 18 is deposited over relatively sharp corners. In some situations such seams may lead to leakage and device failure. In other situations even though the seams may not lead to device failure, the existence of the seams may make a semiconductor manufacturer wary of the integrity of the silicon nitride layer.
Accordingly, new and improved processes for forming uniform, high quality layers of silicon nitride material that can be used for the fabrication of semiconductor devices are continuously being sought.
Embodiments of the present invention provide a technique for forming PECVD silicon nitride films. In some embodiments, a mixed-frequency, high temperature PECVD process is utilized to create a high quality silicon nitride layer having highly conformal properties. The highly conformal nature of the film allows it to be deposited over sharp corners that may exist in some integrated circuits without forming an undesirable seam in situations where PECVD silicon nitride layers previously known to the inventors are susceptible to causing device failure. In one embodiment the silicon nitride layer is deposited in an ammonia rich ambient where the ratio of ammonia to silane in the process gas is at least 10:1. Embodiments of the invention are particularly well suited for front-end applications, such as the formation of spacer structures and the formation of contact etch stop layers, but may be used in other applications also as appropriate. Additionally, embodiments of the invention may be used in the fabrication of integrated circuits having minimum feature sizes less than or equal to 0.13 microns.
According to one embodiment of the invention, a method of forming a silicon nitride layer over a substrate disposed in a substrate processing chamber is disclosed. The method flows a process gas comprising silane and ammonia into the processing chamber, wherein a flow ratio of said ammonia to said silane in said process gas is at least 10:1. A plasma is formed from the processing gas by applying high and low frequency RF power to one or more electrodes in the chamber to deposit the silicon nitride layer.
In one embodiment the process gas used to deposit the silicon nitride layer further includes molecular nitrogen. In other embodiments, during deposition of the silicon nitride layer, the substrate is maintained at a temperature between about 500-580xc2x0 C. and/or the pressure within the chamber is maintained at between about 1.0-4.0 Torr. In some embodiments the silicon nitride film is deposited over a transistor gate as a barrier layer prior to the formation of a premetal dielectric layer. And in still another embodiment, the silicon nitride film exhibits a conformity of at least 90%.